Steroid receptors in the myometrium during pregnancy in the ewe

Three studies were conducted to investigate concentrations of estrogen and progestin receptors in ewe myometrium during gestation. In the first two studies, concentrations were found to be similar in pregnant and nonpregnant ewes on d 5, 9, 12 and 14 postestrus. The receptor concentrations were relatively high on d 5 and were lowest on d 14. Apparently concentrations of myometrial steroid receptors during the luteal phase are commensurate with the successful establishment of pregnancy. The concentration of both types of receptors increased from d 14 to 35 in pregnant ewes. This change was associated with a concomitant increase in the amount of DNA and protein in cells. In Exp. 3, it was found that the concentrations of the estrogen receptor remained fairly constant after d 45 until late gestation, while the concentration of cytoplasmic progestin receptor increased after d 65. The ratio of RNA to DNA appeared to increase throughout pregnancy. Progestin receptor increases may be involved in the continued growth and quiescence of the uterine myometrium.